Page 137 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes Originally Gaussian frequency-shift keying (GFSK) modulation was the only modulation scheme
available; subsequently, since the introduction of Bluetooth 2.0+EDR, /4-DQPSK and 8DPSK
modulation may also be used between compatible devices. Devices functioning with GFSK are
said to be operating in basic rate (BR) mode where an instantaneous data rate of 1 Mbit/s is
possible. The term Enhanced Data Rate (EDR) is used to describe /4-DPSK and 8DPSK schemes,
each giving 2 and 3 Mbit/s respectively. The combination of these (BR and EDR) modes in
Bluetooth radio technology is classified as a “BR/EDR radio”.
Bluetooth is a packet-based protocol with a master-slave structure. One master may communicate
with up to 7 slaves in a piconet; all devices share the master’s clock. Packet exchange is based on
the basic clock, defined by the master, which ticks at 312.5 µs intervals. Two clock ticks make up
a slot of 625 µs; two slots make up a slot pair of 1250 µs. In the simple case of single-slot packets
the master transmits in even slots and receives in odd slots; the slave, conversely, receives in
even slots and transmits in odd slots. Packets may be 1, 3 or 5 slots long but in all cases the master
transmit will begin in even slots and the slave transmit in odd slots.
Bluetooth provides a secure way to connect and exchange information between devices such as
faxes, mobile phones, telephones, laptops, personal computers, printers, Global Positioning
System (GPS) receivers, digital cameras, and video game consoles.

Uses

Bluetooth is a standard wire-replacement communications protocol primarily designed for low
power consumption, with a short range (power-class-dependent, but effective ranges vary in
practice; see table below) based on low-cost transceiver microchips in each device. Because the
devices use a radio (broadcast) communications system, they do not have to be in visual line of
sight of each other, however a quasi optical wireless path must be viable.

Class (m) Maximum permitted power (mW) Range (dBm)
Class 1 100 20~100
Class 2 2.5 4~10
Class 3 1 0~5

The effective range varies due to propagation conditions, material coverage, production sample
variations, antenna configurations and battery conditions. In most cases the effective range of
class 2 devices is extended if they connect to a class 1 transceiver, compared to a pure class 2
network. This is accomplished by the higher sensitivity and transmission power of class 1
devices.
While the Bluetooth Core Specification does mandate minimums for range, the range of the
technology is application specific and is not limited. Manufacturers may tune their
implementations to the range needed to support individual use cases.

8.10 Summary

Stop and Wait protocol is easiest to implement and proves to be the most efficient on an
error free communication channel. However, an error free communication channel is
practically not possible.

PAR is also reliable and easy to implement but at the cost of the loss in bandwidth.
Go Back N protocol needs buffer maintenance and therefore, is complicated to keep source
and destination machines in synchronization. It is also considered the most inefficient
because it retransmits all subsequent frames on the loss of a frame and thus incurs huge
wastage of bandwidth.

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